1. Field of the Invention
The present invention relates to an information recording device for recording or reproducing information magnetically or optically, for example, and, more particularly, to a disk system equipped with sub-actuators for driving a plurality of heads in fine or short strokes independently of one another, in addition to a main actuator for driving those heads together in a long stroke.
2. Description of the Prior Art
Generally, a typical magnetic disk system includes a plurality of magnetic disks on which information is to be stored (recorded), and is equipped with a plurality of magnetic heads for writing and reading information on and from those magnetic disks, and a main actuator such as a voice coil motor for positioning the magnetic heads.
More specifically, the plurality of heads are mounted on a carriage via respective head arms so that the heads are driven together by the voice coil motor. In this case, the heads are usually positioned in accordance with the so-called servo-plane servo system arranged such that any one of the disk surfaces is defined as a servo surface storing thereon positioning information, other disk surfaces are defined as disk surfaces storing thereon data information, and the heads for the data surfaces (i.e., the data heads) are positioned based on the positioning information derived from the head for the servo surface (i.e., the servo head).
However, the above mentioned system has a problem of the so-called "thermal off-track" that the servo head and the data heads are offset due to a difference in coefficient of thermal expansion between respective component parts when there occurs a temperature difference between the points in time of writing and reading information, or that the similar offset takes place due to uneven temperature distribution during operation.
Correction of such thermal off-track becomes increasingly important because the gap between adjacent storage tracks has narrowed in recent years with an increase in the recording density.
As one means for achieving the correction of the thermal off-track, Japanese Unexamined Patent Publication No. 51-36924 or No. 62-75988, for example, discloses a system which includes, in addition to a main actuator (e.g., a voice coil motor) for moving a plurality of heads together in a long stroke, subactuators (e.g., piezoelectric elements) for finely positioning the individual heads independently of one another.
However, the magnetic disk unit is subjected to severe operating and environmental conditions in which acceleration of the head access driving is so very so large as to frequently apply acceleration on the order of more than several tens G to the head section, and the internal temperature is changed over a wide range of from several .degree. C. to several tens .degree. C., for example, dependent on the rest or continuously working state. However, piezoelectric elements are known to be extremely strong against compressive stress, but weak against tensile, bending and shearing stresses. For this reason, special care is needed to ensure reliability in the case of using piezoelectric elements.
Further, for positioning the heads with high accuracy, it is also needed to sufficiently take into account the characteristics of piezoelectric elements.
The prior art has not paid thorough consideration to those points and still has room for improvement from the standpoint of practical use.